Abstract

The picosecond geminate recombination of oxygen in hemoglobins is highly responsive to the tertiary structure of the heme environment.¹ In particular, the geminate yield changes in response to changes in the heme-proximal histidine geometry as reflected in the frequency of the iron-proximal histidine stretching mode (ν (Fe-His)) . Non-exponential kinetics for the ps geminate recombination are observed.¹ Since ν(Fe-His) is the same at 30 ps and 10 ns subsequent to photodissociation, the non-exponential kinetics cannot originate from a subns structural diffusion involving the heme histidine geometry. One alternative explanation is that the dynamics associated with conformational heterogeneity are sufficiently slow that the initial geminate recombination reflects preferential binding to specific substate conformations. Because of the suggested relationship between ν(Fe-His) and the innermost barrier controlling geminate rebinding,³ we have monitored the lineshape of ν(Fe-His) of the surviving population of the deoxyheme photoproduct during the picosecond geminate rebinding in order to determine whether this Raman band is inhomogeneous broadened with respect to functional selectivity. Our preliminary results reveal only a monotonic decrease in the intensity without any change in lineshape. Thus it appears that for ligand binding dynamics that there is no indication of conformational heterogeneity on the 10's of ps time scale at least for the heme-histidine geometry. A similar study at cryogenic temperatures is currently in progress.

© 1986 Optical Society of America